1. Field of the Invention
The present invention relates to a general surface monitoring method which uses samples prepared for multiple internal reflection and subsequent spectroscopy to detect the surface condition of the sample. A particular application of the method is for in-situ monitoring, by Fourier Transform Infrared Spectroscopy using Multiple Internal Reflectance (FTIRS-MIR), of surface contamination or lack of hydrogen passivation on a silicon sample interrupted at any point in the manufacture of semiconductor devices prior to film growth or deposition by physical or chemical means (such as epitaxy).
2. Description of the Prior Art
Fourier Transform Infrared Spectroscopy using Multiple Internal Reflections (FTIRS-MIR) is a well-known technique to improve the signal to noise ratio (S/N) by multiple interactions with the sample surfaces for a given short period of time. This allows a real-time surface monitoring capability at the monolayer level.
Y. J. Chabal, G. S. Higashi and K. Raghavachari in "Infrared spectroscopy of Si(111) and Si(100) surfaces after HF treatment: Hydrogen termination and surface morphology", Journal of Vacuum Science and Technology, May/June 1989 (2104-2109), found that upon HF treatment, the surfaces of both Si(111) and Si(100) are microscopically rough with mono-, coupled mono-, di- and trihydride termination. Si(111) surface forms a regular array of double-layer steps with evidences for a dimer reconstruction of the step atoms. The trihydride interacts with the steps in a manner that breaks the degeneracy of the asymmetric strength.
Kaneta et al., U.S. Pat. No. 5,066,599, Silicon Crystal Oxygen Evaluation Method using Fourier Transform Infrared Spectroscopy (FTIR) and Semiconductor Device Fabrication Method using the Same, describe a technique of measuring oxygen impurities in silicon crystals. In this technique several measurements must be taken to calculate the absorption peaks of oxygen impurity in the silicon crystal. For this reason very small vibrational reflections cannot be detected.
Takizawa et al., Japanese laid open application No. 63-157343, Measuring Method for Impurity Concentration of P-type Silicon Crystal, demonstrate another technique for measuring impurities in silicon crystals. In their procedure, an intermediate crystal is sandwiched between two highly doped P-type silicon crystals, and the highly doped P-type silicon crystals are irradiated by an infrared light ray which is reflected between the two crystals through the intermediate crystal. The infrared spectra of the exiting light ray is used as a measure of the impurity in the highly doped P-type silicon crystals.
Sting, U.S. Pat. No. 4,595,833, Multiple Internal Reflection Cell Optical System for use in Infrared Spectrophotometry of Liquid and Fluidized Samples, uses reflaxicon optics for directing infrared radiation from a source into the entry and directs radiation from the exit to a detector. This invention is best used on liquids and fluidized samples. This method is not useful for evaluating impurities in silicon wafers without destroying their structure.
Passivation of silicon surfaces prior to film formation on such surfaces is essential to prevent oxide or carbide formation. The passivation must be maintained from the point of cleaning, through the loading and up to the time the gases for deposition are admitted to the system. There has been variability observed in both the Ultra-High Vacuum (UHV), Low Temperature Epitaxy (LTE), and Medium Temperature Epitaxy (MTE) film growth quality and epitaxial interface contaminants. Oxygen, carbon and fluorine have been observed by Second Ion Mass Spectrometry (SIMS) even though surfaces were passivated with hydrogen via a hydrofluoric acid (HF) final rinse step.
For the MTE case (850.degree. C. films), the HF final rinse step was also shown to be beneficial in reducing oxide and carbon content and corresponding film improvements were observed with these reductions. Variability and reproducibility in both epitaxial processes is troublesome, however.
It has been demonstrated that various surface hydrides are strongly dependent on the hydrogen exposure and the adsorption temperature. In general, the higher hydrides (SiH.sub.2 and SiH.sub.3) are formed at high hydrogen exposures. A general trend has been observed that the formation of higher hydrides is favored by low adsorption temperature. The apparent sticking coefficient as well as the saturation coverage of hydrogen on Si(111)-7.times.7 are temperature dependent, resulting from a series of competing surface reactions with different temperature dependencies. The existence of tilted monohydrides and SiH.sub.2 and SiH.sub.3 species on the surface show that an extensive breaking of Si--Si bonds must occur during hydrogen adsorption. The bond breaking induces a disordering of the Si(111)-7.times.7 surface.